People with inadequate renal function, hypoparathyroidism, or certain other medical conditions often have hyperphosphatemia, or elevated serum phosphate levels (over 6 mg/dL). Hyperphosphatemia, especially if present over extended periods of time, leads to severe abnormalities in calcium and phosphorus metabolism, often manifested by hyperparathyroidism, bone disease and calcification in joints, lungs, eyes and vasculature. For patients who exhibit renal insufficiency, elevation of serum phosphorus within the normal range has been associated with progression of renal failure and increased risk of cardiovascular events. The progression of kidney disease can be slowed by reducing phosphate retention. Thus, for renal failure patients who are hyperphosphatemic and for chronic kidney disease patients whose serum phosphate is within the normal range or is only slightly elevated, therapy to reduce phosphate retention is beneficial.
For patients who experience hyperphosphatemia, calcium salts have been widely used to bind intestinal phosphate and prevent its absorption. Different types of calcium salts including calcium carbonate, acetate, citrate, alginate, and ketoacid salts have been utilized for phosphate binding. The major problem with all of these therapeutics is the hypercalcemia which often results from absorption of high amounts of ingested calcium. Hypercalcemia causes serious side effects such as cardiac arrhythmias, renal failure, and skin and visceral calcification. Frequent monitoring of serum calcium levels is required during therapy with calcium-based phosphate binders. Other, calcium and aluminum-free phosphate binders have drawbacks including the amount and frequency of dosing required to be therapeutically active.
An alternative approach to the prevention of phosphate absorption from the intestine in patients with elevated phosphate serum levels is through inhibition of the intestinal transport system which mediates phosphate uptake in the intestine. It is understood that phosphate absorption in the upper intestine is mediated at least in part by a carrier-mediated mechanism which couples the absorption of phosphate to that of sodium in an energy-dependent fashion. Inhibition of intestinal phosphate transport will reduce serum phosphate levels. This would be particularly advantageous in patients susceptible to hyperphosphatemia as a result of renal insufficiency or in patients that have a disease that is treatable by inhibiting the uptake of phosphate from the intestines. Inhibition of phosphate absorption from the urine by the kidneys would also be advantageous for treating chronic renal failure. Furthermore, inhibition of phosphate transport may slow the progression of renal failure and reduce risk of cardiovascular events.